Charge Coupled Device (CCD) based cameras are commonly used as input devices for image analysis systems. A disadvantage of this class of devices is that when real-time operation is required, only simple calculations can be performed on the CCD output data stream.
A prior art CCD image sensing device is described in U.S. Pat. No. 5,335,008 ('008 patent). The '008 patent is directed to a CCD image sensing device in which an amplifying transistor for amplifying a photoelectrically-converted signal charge and outputting the amplified signal charge to a vertical signal line is provided at every pixel of a plurality of pixels arrayed in a two-dimensional fashion and wherein a load transistor is connected to each of the vertical signal lines, in which the load transistor is composed of a first field effect transistor having a constant current characteristic and a second field effect transistor acting as a negative feedback resistor function connected in series.
In this description, it is the first MOSFET transistor which is reacting as the load of a signal line and serving therefore as the load of a source follower. Accordingly, this transistor has no direct relation with the signal read from the photodiode.
Furthermore, this document describes a CCD image sensing device wherein the first MOSFET transistor has a large ratio W/L wherein W is its channel width and L is its channel length. However, it should be noted that such W/L ratio directly affects the gate voltage (V.sub.G -V.sub.th) of the MOSFET for a given current. The aim of making this characteristic precise suggests the use of MOSFET transistors as linear resistance elements. The voltage (V)/current (I) relationship for MOSFET transistors is given by the equation I=.mu..sub.n C.sub.ox (W/L)(V.sub.G -V.sub.TH)V.sub.D. Accordingly, to achieve linear operation it is necessary to work in the region where "V.sub.G &gt;V.sub.th."
CMOS based flexible imaging sensors have been developed recently (S. Anderson, IEEE 1991, Custom Integrated Circuits Conference, pp. 12.1.1-12.1.4). Examples of these sensors include the IMPUTER of the VLSI Vision Ltd. company (Scotland) and the MAP2200 product of Integrated Vision Products (Sweden).
A classic image sensor of the integrating type and having a particular source follower circuit is described in European Patent Application No. 92122002.6, entitled "Source Follower Circuit for Image Sensor," Publication No. 0 548 987 ('987 patent). The '987 patent discloses an image sensor in which each pixel in the image sensor comprises one MOSFET of a complex nature such that it includes light sensitive means. In this device, the photo current is accumulated on a capacitance and grows during illumination. At the end of "integration time", it must be reset. The same principle is found in many diode arrays, CCD's devices, MOSFET cameras, etc.
An imaging chip intended for use in a CMOS image sensor with a large number of image pixels with field-effect transistors and a read-out logic is disclosed in PCT Patent No. WO93/19489 (PCT patent). As disclosed in the PCT patent, to map a high input signal dynamic ratio onto a reduced output signal dynamic ratio, each image pixel is connected to one electrode of a first MOS transistor and to the gate of a second MOS transistor while the other electrode of the first MOS transistor is connected to one pole of a voltage source.
An image array of 256 by 256 pixels developed in a 2.4 micron CMOS technology is disclosed in Microelectronics Engineering, Vol. 19 (1992) pp. 631-634 (ME Vol. 19). ME Vol. 19 discloses an image array which acts as an asynchronous component having as control signals of one pixel, two 8 bit input words forming the address of said pixel and one analog output. This device is fully addressable which implies that the pixels can be read out in a true random sequence. Furthermore, the pixels can be read out instantaneously, which is a consequence of the fact that in this device architecture the detected photocurrent is continuously converted to a low impedance voltage signal within the pixel. A general view of the device architecture of the present invention will be described in detail below and more particularly, in connection with FIGS. 1 and 2.
The device described in ME Vol. 19 may function as a compact, low cost smart vision sensor, e.g. for industrial imaging purposes. The device concept allows co-integration with digital logic, in order to build smart vision cameras. The logarithmic response causes this sensor to respond to a broad range of illumination conditions, with nearly perfect anti-blooming performance.
The combination of addressability and continuous, asynchronous readout characteristics allows the read out of the response of any pixel at any time. Both characteristics are intimately linked, adaptations in device architecture to improve one of these two characteristics has a direct beneficial effect on the other characteristic. By using the above described technology, one can achieve a sensor having improved addressability characteristics or continuous readout characteristics.
The CMOS image sensors as disclosed in the above-mentioned references have shortcomings in applications such as industrial imaging. The main disadvantage of the circuits as described in the prior art is that the quality of the image is inferior to CCD system of the present invention. The light sensitivity of prior art imaging devices is limited by constraints inherently linked with the implementation of CMOS technology in their circuits. Also, in prior art device architectures, an absolute measure of the light intensity cannot be provided. Further, a brightness control of the image cannot be achieved by state-of-the-art techniques implemented in CCD's or imaging devices based on integrating techniques of the prior art.
Additionally, in prior art imaging devices, the homogeneity of the image is inherently degraded by the statistical spread on the individual pixel characteristics. Defective "white pixels" can degrade the image homogeneity as well. A further disadvantage of the prior art imaging devices is that all such sensors have only black and white contrast and, accordingly, no color sensitivity.